The present invention relates to a seat belt pretensioner and, in particular, to a pretensioner in which a length of the seat belt is wound onto a reel that is driven by a rotary piston/cylinder drive mechanism supplied with a gas under pressure from a gas generator.
It is well-known to equip vehicle seat belt systems with belt pretensioners that tighten a loosely fitting belt firmly against the occupant in a collision and more effectively restrain the occupant in the seat. Among the various types of pretensioners that have been proposed heretofore are those in which the reel of a seat belt retractor is rotated in the belt-winding direction to wind a length of the belt onto the reel by a turbine, a gear motor, or a rotary piston/cylinder powered by a gas under pressure supplied by a gas generator. The following references disclose seat belt pretensioners of this type:
Japanese Patent Publication No. 55-13736 ("Ref. 1");
Japanese Patent Publication No. 59-15657 ("Ref. 2");
Japanese Patent Laid-open Publication No. 54-142730 ("Ref. 3");
Japanese Patent Laid-open Publication No. 54-142731 ("Ref. 4");
Japanese Patent Laid-open Publication No. 60-45450 ("Ref. 5"); and
Japanese Patent Laid-open Publication No. 60-45449 ("Ref. 6");
When a belt pretensioner is built into a retractor, provision must be made for the retractor to operate normally in allowing the belt to be unwound from and wound onto the reel. Accordingly, the drive mechanism of the pretensioner cannot be engaged with the reel until the pretensioner is operated, and a device must be incorporated to connect the drive mechanism to the reel when the operation of the pretensioner is initiated.
Ref. 1 proposes a turbine impeller driven by gas under pressure to rotate the retractor reel, and while referring generally to a projecting fixed shaft, a slip joint or a friction coupling to connect the impeller to the reel, does not describe or show in detail such couplings. Ref. 2 discloses coupling the piston of a rotary piston/cylinder to the flange of a belt reel by a pin on the piston that is normally retracted into the cylinder chamber but is pushed by the gas pressure upon operation of the piston/cylinder into one of a series of holes arranged in a circle around the reel flange. In Refs. 3 and 4 rotating impeller is directly coupled to a reel, and there are no descriptions of how the retractor operates normally. The rotary piston/cylinder of the pretensioner of Refs. 5 and 6 is coupled to the reel shaft upon operation of the pretensioner by a ball and cam clutch mechanism.
The mechanism of Refs. 5 and 6 is of complicated design, requiring many parts and intricate assembly, and involves a series connection from the rotary piston/cylinder to the clutch to the reel, which undesirably increases the size of the retractor in the direction of the reel shaft. The mechanism of Ref. 2 is of a simpler design but is also of large size. Moreover, the pin on the piston does not normally register with and move into a hole in the reel flange until the piston has rotated through an idle distance, and the engagement is not certain and reliable because of the idling motion.